git-gui: Allow gitk to be started on Cygwin with native Tcl/Tk

git-gui: Allow gitk to be started on Cygwin with native Tcl/Tk

gitk expects $env(GIT_DIR) to be valid as both a path that core Git
and Tcl/Tk can resolve to a valid directory, but it has no special
handling for Cygwin style UNIX paths and Windows style paths.  So
we need to do that for gitk and ensure that only relative paths are
fed to it, thus allowing both Cygwin style and UNIX style paths to
be resolved.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>